btrfs: move btrfs_page_mkwrite() from inode.c into file.c

btrfs: move btrfs_page_mkwrite() from inode.c into file.c

btrfs_page_mkwrite() is a struct vm_operations_struct callback and we
define that structure in file.c. Currently the function is in inode.c and
has to be exported to be used in file.c, which makes no sense because it's
not used anywhere else. So move btrfs_page_mkwrite() from inode.c and into
file.c.

While at it do a few minor style changes:

1) Capitalize the first word of every comment and end each sentence with
   punctuation;

2) Avoid splitting some statements into two lines when everything fits in
   85 characters or less.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>