Btrfs: use mnt_want_write_file instead of mnt_want_write
Btrfs: use mnt_want_write_file instead of mnt_want_write

mnt_want_write_file is faster when file has been opened for write.

Signed-off-by: Liu Bo <00b627b8656913e4df3ede997ad1311ebf4b82a4@cn.fujitsu.com>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas