PCI/sysfs: Fix 'resource_alignment' newline issues

PCI/sysfs: Fix 'resource_alignment' newline issues

The value of the "resource_alignment" can be specified using a kernel
command-line argument ("pci=resource_alignment=") or through the
corresponding sysfs attribute under the /sys/bus/pci path.

Previously, when the value was set via the kernel command-line argument,
and then subsequently accessed through sysfs attribute, the value read back
was not correct:

  # grep -oE 'pci=resource_alignment.+' /proc/cmdline
  pci=resource_alignment=20@00:1f.2
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.

This was also true when the value was set through the sysfs attribute
without including a trailing newline:

  # echo -n 20@00:1f.2 > /sys/bus/pci/resource_alignment
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.

When it was set through the sysfs attribute *including* a newline,
reading it back worked as intended:

  # echo 20@00:1f.2 > /sys/bus/pci/resource_alignment
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.2

To fix this inconsistency, append a trailing newline in the show() function
and strip the trailing line in the store() function if one is present.

Also, allow for the value previously set using either a command-line
argument or through the sysfs object to be cleared at run-time.

[bhelgaas: fold in kfree fix from
https://lore.kernel.org/linux-pci/20210604133230.983956-4-kw@linux.com]
Fixes: e499081da1a2 ("PCI: Force trailing new line to resource_alignment_param in sysfs")
Link: https://lore.kernel.org/r/20210603000112.703037-4-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>